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Armor penetration vs physical damage runes

Well i read alot for this both runes and i saw alot of builds .. and they all just stack only armor penetration runes.Well for me this is not the best stats for carry.I mean physical damage runes give u more buffs than armor pen.They buff u basic atack + skills.Armor penetration buffs u only ur basic atack.. and if i read right armor pen runes + talant cant reach bellow 0 armor.Correct if i`m wrong but i think the best runes for melle carry or range... is physical runes not armor pen. (sorry for my bad english)

1) With a base damage of like 60 and +16 from runes ,you will deal ~76 damage. But IF the enemy champ has more than like 20 armor (which is ~18% dmg reduction or something like that) you will deal -13 damage. Which means with a base damage of 76 ,you will be dealing 63 per hit..... To squishies like Ashe.

2)15 Armor pen + 7 Ad. 15 ArP against a target with like 20 Armor (which is lets say ~18% dmg reduction) means you will neglect like 13% of his armor ,maybe even more ,not sure.
So if you have 67 base AD (your champ's + the Quints) and do only ~3 - 5% less damage because of the enemys armor ,that leaves you at ~62-3 damage per hit.

Now lets see another scenarion.

When the target has more than 20 armor ,the damage reduction from armor is even greater ,thus making ArP runes even better.

ArP is just better.

AND it also affects physical damage abilities

Now if you add an AD item like BF or Dorans Blade to the calculations above you will understand what Im talking about.

Well i read alot for this both runes and i saw alot of builds .. and they all just stack only armor penetration runes.Well for me this is not the best stats for carry.I mean physical damage runes give u more buffs than armor pen.They buff u basic atack + skills.Armor penetration buffs u only ur basic atack.. and if i read right armor pen runes + talant cant reach bellow 0 armor.Correct if i`m wrong but i think the best runes for melle carry or range... is physical runes not armor pen. (sorry for my bad english)

You are actually wrong - that's not my opinion, there is math (:O) and logic behind this claim.
First of all, most AD carry skills deal physical damage, which benefits from Armor Penetration the same way autoattacks do.
Armor can actually become negative, in which case it counts as damage amplification, not damage reduction.
Thus, the best runes are the Armor Penetration ones. With max ARPEN runes, you get ~26 ARPEN at lv1. With max AD, you get ~16 AD.
Even assuming that your spells have an AD ratio of 1.0 or higher (which is not the case more often than not), and taking into account that this extra AD is considered for skills that scale off of bonus AD, ARPEN is still better. Why?
Here is the reason:
Assume you are playing a champion with base 60 damage at lv1. You use AD runes, so that goes up to 76 at lv1. Your autoattack wants to deal 76 damage, but the enemy has 30 armor (typical value for NO armor runes on the enemy, with runes it's about 42). Thus, the formula that gives the final damage output is:
-- Damage Multiplier = 100 / (100 + Armor) when armor ≥ 0
Thus, the damage taken in the end will be reduced by 23% (that's about one quarter), so the enemy will take 58 (58,46...) damage out of your 76 AD!
If you have 26 ARPEN though, the enemy is reduced to 4 armor. Thus, the damage is now reduced by a mere 4%, meaning the opponent takes 58 (57,69...) damage from your attack.
So the end result is the same, right? Nope! If you add more damage to a build with AD runes, the damage reduction by armor will remain the same - so you'll actually be dealing much less damage than your AD - if you use ARPEN runes though, your damage is decreased by a much smaller amount, thus your overall damage is higher with ARPEN.
Example:
Say you have 116 damage with AD and 100 with ARPEN. Enemy has 30 armor:
AD: 89 damage
ARPEN: 96 damage
So with LESS damage, you deal MORE damage :O

As a bonus, let's see what happens when you have A LOT of ARPEN, say:
26 from runes
20 from Yomuu's Ghostblade
15 from Brutalizer
20 from Stark's Fervor
45 from Black Cleaver
-------------------------------- Total= 126 ARPEN.
Suppose the enemy had 60 armor, cause they had armor runes and bought cloth armor.
With -126, they now have -86 armor. NEGATIVE 86 armor.
The formula here is:
-- Damage Multiplier = 2 − 100 / (100 − Armor) when armor ≤ 0
Thus, assuming you have 200 damage (with all those items you might actually have more), that means the damage multiplier is 1.46, or that your damage output is INCREASED by 46% !
So, an autoattack now deals (with 200 AD) 292 damage!!!!

Even if you had an AD user with some ARPEN (which brought enemy armor back to 30 from 60) with 300 damage (100 more than the ARPEN build, and you still get some ARPEN), the damage output would still be reduced by 23%, meaning you'd deal 230 damage! So with 100 more damage, and with 30 ARPEN, you deal 62 less than the full ARPEN build!

As a bonus, let's see what happens when you have A LOT of ARPEN, say:
26 from runes
20 from Yomuu's Ghostblade
15 from Brutalizer
20 from Stark's Fervor
45 from Black Cleaver

this only stacks if you target has at least 126 armor
armor reduction can get some1 to below zero armor
armor penetration cant
how its calculated:
-first you reduce by any % armor reductions (wukongs Q/urgots DoT/nidalee trap) these are multiplied (40% reduction from nida * 30% reduction from wukong = ~57% armor reduction)
-then you reduce remaining armor by any flat armor reductions (starks/tarics shield shater/nasus fancy circle/karthus wall) these reductions are additive
-then you count in your Armor Pen
-then you count in your % penetration (only from last wisper)
source: http://leagueoflegends.wikia.com/wiki/Armor_penetration

as for the original poster

most non casters start with about 30 armor (you can have 31 armor pen by runes + masteries =ignoring their armor for fist few lvls) and casters are between 10-16
BUT armor is a stat that improves by lvl up thus meaning by the time you are carried (since you play the carry) they could have more armor thus making ArP (a stat that can be aquired in game by 1 item only [brutaliser->yumus] really important)

Well i read alot for this both runes and i saw alot of builds .. and they all just stack only armor penetration runes.Well for me this is not the best stats for carry.I mean physical damage runes give u more buffs than armor pen.They buff u basic atack + skills.Armor penetration buffs u only ur basic atack.. and if i read right armor pen runes + talant cant reach bellow 0 armor.Correct if i`m wrong but i think the best runes for melle carry or range... is physical runes not armor pen. (sorry for my bad english)

Well after the removal of vamp scept opener (it costs double the price now) actually lifesteal quints are the most recommended, and every vayne out there uses that. You may wish to go for attack speed quints or attack damage quints also. Depends on the ADC and personal choice. If you have trouble lasthitting with some ADC use AD quints until you get used to his animation. ARpen quints are something that will gimp your early game even more. You cant rely on support so get either AD or lifesteal quints. Well you can and thats the point of support but you are yet to meet the dorans - a nice family who never buys wards. MS quints work well on a few ADCs but only if you know them well and can play at disadvantage early (blue ezra build for example).
As for dmg itself - it was possible to have minions at minus magic res last season with the changes to penetration mechanics http://leagueoflegends.wikia.com/wiki/Armor_penetration 3. and 4. were switched in order before, and now penetration overall is more powerful. I never tried bringing something at -armor, but it worked for -mr. Anyway very few champions will forgo armor runes and items completely for you to possibly bring them to minus. You can also find some graphs at reignofgaming and mobafire forums that show the exact breakpoints of AD when beginning to stack penetration starts to really pay off the most.
TLDR lifesteal or ad quints.